T. cruzi clones and inbred mice are being used to develop experimental models for Chagas' disease. These studies demonstrate that parasite genetics plays a major role in the course and outcome of an experimental T. cruzi infection. In addition, they demonstrate that the parasite population obtained from chagasic patients are heterogenous. Dose-response studies show that the size of the parasite inoculum modulates the course of a T. cruzi infection but it does not change substantially its final outcome. In addition, the long-term stability of two T. cruzi clones was demonstrated by their pathogenicity for C3H mice following various in vitro manipulations of the parasites.